The present invention relates to a device for mooring a floating installation to an offshore installation having a substantially stationary position relative to the water bottom.
The floating installation may be a ship and the offshore installation a mooring buoy located at some distance from the shore.
The offshore installation may also be a buoy for loading and/or unloading the cargo of a ship or installation wherefrom are performed or controlled such operations as drilling the sea bottom, recovering oil products, or alternatively a buoy, a floating caisson a tank, etc.
These type of systems are already known for mooring a ship to a structure such as a buoyant caisson, or to an oscillating riser pipe, by means of pivoting arms.
Such systems are, for example, described in the French Pat. No. 1,403,493 and the U.S. Pat. Nos. 3,354,479 and 3,908,212.
In such systems, the mooring structure is surmounted by a rigid arm integral with the ship in the extension of its stem on which it is articulated so as to accomodate pitching movements. To permit rotation of the arm and of the ship about the vertical axis of the mooring structure the arm is connected to this structure by means of a pivot provided with ball bearings or roller bearings, this pivot making up a rotary table or orientation ring.
Such prior devices suffer from many drawbacks with regard to resistance, reliability and constructional yield.
First such rotary tables which are very sensitive to marine corrosion are, like all roll bearings, designed for fast rotation and for successive and similar operation of all their parts. Moreover, they are poorly adapted to solve the problem of slow and occasional rotation where alternating forces developed by swells are exerted most of the time without substantial rotation of the table.
Since the prevailing winds statistically preferentially maintain the ship in one heading and consequently the connecting arm, the same area of the rotary table and thus, the same balls of the bearing are permanently maintained under load.
This mode of operation requires such an oversizing of these parts as to cause problems at the present time in the construction of large installations.
Moreover, in such a device loads of generally very high values are concentrated by the rotary table at the level of the central part of the upper platform of the mooring structure. This requires extensive reinforcements for the purpose of distributing all these loads over the entire structure.